1. Field of the Invention
The present invention relates to a fluorescent x-ray analyzer and, more particularly, to an improved fluorescent x-ray analyzer that can monitor the status of the x-ray tube and thereby increase the effective life of the instrument, while ensuring accurate readings.
2. Description of Related Art
Fluorescent x-ray instruments have been utilized as analytical instruments. Reference can be made to FIG. 4 to disclose a schematic construction of one form of a fluorescent x-ray analyzer. In this regard, a sample can be held on a sample monitoring stage (not shown) and subjected to irradiation from primary x-rays 3 from an x-ray tube 2. As a result, fluorescent x-rays and scattered x-rays 6 are generated at the sample, and a filter 4 is placed before an x-ray detector 5. An output signal from the x-ray detector is processed in a pulse height analyzer (not shown) after suitable amplification to conduct a predetermined analysis.
FIG. 5 discloses one form of construction for controlling the output of the x-ray tube 2. The x-ray tube 7 supports a vacuum and contains a thermal cathode 10 that includes a filament 8 and a cathode 9 that is connected to an appropriate power source so as to generate thermal electrons 11. The cathode 9 is connected through a buffer amplifier 12 to the input terminal 13a of a comparator 13. An x-ray tube electric current I.sub.x may flow through a detecting resistance 14 provided on the input side of the buffer amplifier 12, to thereby generate a voltage V.sub.x obtained by converting the x-ray tube electric current I.sub.x into a voltage value. This voltage V.sub.x is input as one signal to the comparator 13.
A target 16 is mounted at the other end of the tube member 7 as an anode, and it is connected with a high-voltage power source 15. An x-ray transmissive window 17 made, for example, of beryllium is formed and provides an output from the tube 7 of the primary x-ray 3. A first grid member 18 is capable of regulating the quantity of thermal electrons 11 that are permitted to collide with the target 16. The quantity of thermal electrons 11 is a function of the x-ray tube electric current I.sub.x, and the grid 18 can provide a constant value of control thermal electrons 11. A second grid member 19 is used for contracting thermal electrons before they collide with the target 16 so that the stream of electrons is not excessively expanded and are controlled to be arranged between the thermal cathode 10 and the target 16.
A controlled set value for regulating the x-ray tube electric current I.sub.x can be input by the operator into the other input terminal 13b of the comparator 13 as the voltage signal V.sub.R. This voltage signal V.sub.R is compared with the voltage signal V.sub.x in the comparator 13 to provide a feedback loop to apply a voltage to the first grid 18 through a level converter circuit 20. As a result, a controlled grid voltage of the first grid 18 can be desirably controlled so as to provide a predetermined x-ray tube electric current I.sub.x.
A problem that can impact on the use of fluorescent x-ray instruments has been the stability and life of the x-ray tube 2. The inside of the tube member 7 can deteriorate in degree of vacuum where the thermal cathode 10 can deteriorate to produce an emitting factor of the thermal electrons 11. As a result, the ability to provide constant current control deteriorates, and eventually can become impossible. In the conventional fluorescent x-ray analyzer, it becomes difficult to determine the specific time period in which an x-ray tube becomes inaccurate or its control current starts to deteriorate. As a result, erroneous readings can occur as the quantity of x-rays emitted by the x-ray tube 2 is reduced. As can be appreciated, when the x-ray tube 2 loses its ability to be controlled by the operator, it is necessary to exchange the x-ray tube 2. The life of the x-ray tube 2, however, cannot be readily determined. The prior art has frequently resorted to periodic changes of the x-ray tube 2 to guard against analytical errors. As can be appreciated, however, the life of an x-ray tube 2 could be extended beyond the periodic changing, since the maintenance schedule usually requires a safety factor to avoid erroneous readings. Thus, the cost of x-ray tubes 2 must be increased to cover the wasteful utilization of them in an analytical instrument.
The prior art is still seeking an improved fluorescent x-ray instrument for analytical use.